1. Field of the Invention
The present invention relates to a transmitting apparatus for code division multiplexed signals, and more particularly to a transmitting apparatus which composes multiple pieces of orthogonal-code-converted transmission information, renders the spread spectrum with a spread code for the composed signal, and transmits the resulting wireless multiplexed signal.
2. Description of the Prior Art
A conventional transmitting apparatus for code division multiplexed signals (e.g., the code division multiplexing communication equipment described in publication "TIA/EIA INTERIM STANDARD, TIA/EIA/IS-95-A,7,1.3.1", May 1995) used for the spread spectrum communication has its transmitter section designed such that transmission information 11-i (where i=1,2, . . . n) of multiple channels are scrambled so that a multiplexed signal 21 resulting from the composition of the multiple channels has characteristics close to the white noise as shown in FIG. 8.
Scrambling is carried out by multiplying the transmission information 11-i to scramble signals Si produced by a scrambler 15 with multipliers 12-i, respectively. For the orthogonal transformation of the transmission information, the scrambled transmission information are multiplied to orthogonal codes Wi produced by an orthogonal code generator 14 with multipliers 13-i, respectively. Next, the transmission information of the channels are multiplied to power setup values Ai provided by an information power controller 15 so that each channel has a prescribed power level, and the resulting transmission information of all channels are summed by an adder 18. The output of the adder 18 is multiplied to a spread code (termed simply PN code) provided by a spread code generator 20 with a multiplier 19, resulting in a spread spectrum multiplexed signal 21. The multiplexed signal 21 has its sampling rate raised by an interpolator 23 so that it undergoes bandwidth limition by a digital filter. In the case of raising the sampling rate by 4, for example, the interpolator 23 operates to insert three 0-amplitude samples between the original sampling points. A baseband filter 24 is used to limit the bandwidth of the over-sampled signal. The baseband filter 24 has its output signal converted into an analog signal by a D/A converter 25, converted in frequency to have the carrier frequency by an RF circuit 26, amplified in power by an amplifier 27, and transmitted by an antenna 28.
The above-mentioned conventional transmitting apparatus of the code division multiplex communication equipment uses the scrambler 15 so as to provide the multiplexed signal with characteristics close to the white noise, so that the signals of all channels have less chance of having the same value simultaneously and the amplifier does not have input signal components of excessive amplitudes.
However, the spread code mentioned above is generally a pseudo-random number, and due to the summation of the multiple communication channels following the rendition of spread spectrum, the amplitude of the multiplexed signal 21 has virtually a normal distribution in general. Namely, a signal component with a large amplitude can possibly arise, although the frequency of occurrence is very low. When the analog last-stage amplifier 27 receives an input signal having an amplitude in excess of its linear operation region, the amplifier has its output signal distorted, resulting in a spurious generation outside its own bandwidth. Release of spurious from the antenna 28 is illegal, and therefore it is crucial to design the circuit so that it does not create a spurious. On this account, the analog amplifier 27 is provided with a wide linear operation region, i.e., large back-off. Since the circuit arrangement of FIG. 8 following the D/A converter 25 is configured with analog circuits, providing a large back-off cases the amplifier to have a degraded power efficiency and need a power source of a higher voltage. Moreover, it necessitates an up-sized D/A converter 25 and RF circuit 26 against the intention of low manufacturing cost and compact design of the apparatus.